Jean Hardouin (1646-1729) was a scholar of classical literature. In 1685 he published an edition of Pliny's Natural History. There was nothing unusual about the edition itself, which was considered to be of merit and very well edited. What was unusual was that despite being so knowledgeable about classical literature, Hardouin had very strange ideas about its origins.

According to Hardouin, the majority of classical Greek and Roman literature had not been produced by Greek and Roman authors. Instead, it had been forged during the Middle Ages by a group of Benedictine monks. He also argued that all extant Greek and Roman coins were forgeries. He never revealed why such a vast deception had occurred. He only declared, elliptically, that when he died the reason would be found written on a piece of paper the size of his hand. The reason, unfortunately, was never found.

References:

Mencken, Johann Burkhard. The Charlatanry of the Learned. (De charlataneria eruditorum, 1715). Translated from the German by Francis E. Litz, with notes and an introduction by H. L. Mencken. "First edition." New York, London, A.A. Knopf, 1937.